Incorrect spelling


Incorrect spelling, explanation: we cannot skip v sound in cardinal nor ordinal number twelve (twelfth). It just becomes voiceless in the ordinal form (because of th sound attached). This is why twelth is incorrect and the correct spelling is twelfth.

Correct spelling


Correct spelling, explanation: ordinal number twelfth should be spelled with f because in the cardinal number twelve there is the same, although voiced, sound v. It becomes voiceless because of another voiceless sound th, but it remains. This is why twelfth is the correct form and not twelth.

Definition of twelfth:
numeral, ordinal number between eleventh and thirteenth
They had eleven sons and to everybody’s surprise their twelfth child was a girl.

Incorrect spelling


Incorrect spelling